(1) Field of the Invention
The present invention relates to correlating sonar images and more specifically to systems of sonar transmitters/receivers (also referred to herein as sonars) with overlapping frequencies that observe terrain from overlapping aspects, methods for converting sonar images into forms that can be correlated, and for coherent correlation of the images.
(2) Description of the Prior Art
As is known in the art, coherent sonar image correlation is difficult to achieve. This is generally attributed to the tendency of sonar images to change dramatically in response to small changes in angle. Reasons why sonar images of the same terrain fail to correlate include lack of frequency overlap, lack of aspect overlap, and sonar artifacts.
If an object is observed with different frequencies it will not correlate since correlation involves multiplication in the frequency domain. If the frequency domain extent of one signal does not overlap with the extent of the other, the multiplication will yield no non-zero region, and the signals will not correlate. In order for correlation to occur there must be overlapping frequency bands.
Less intuitive is the effect of aspect angle. Like frequencies, aspect angles also correspond to regions in the frequency domain. When two images that are composed of measurements made from non-overlapping aspects are correlated, their frequency domain multiplication yields no non-zero region, leading to zero correlation. In order for correlation to occur there must be overlapping observation angles.
Unfortunately, observing targets over the same range of angles in three dimensions is difficult, as it requires planar arrays that conform to the open/closed aperture theorem. Therefore, it is usually impossible to achieve vertical or grazing angle overlap. Even if vertical or grazing angle overlap is achieved, the constraint of observing the subject terrain over the same horizontal aspects remains.
What are needed are systems and methods for converting sonar images into a form which can be correlated and for correlating coherent sonar images.